I may be wrong, but to me seems that you have a superseded BootROM version.
Install all the updates, after that check if you already have 4188.8.131.52.0 BootROM release.
Install the AppleOEM 12+16 blade back to the GPU 12+16 socket, power on, do a deep NVRAM reset and then an Internet Recovery fully erasing the AppleOEM blade. Install whatever macOS release Internet Recovery offers, then upgrade to BigSur.
- 6.1 2013 Mac Pro won’t boot after 11.3.1 update
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